1. FIELD OF THE INVENTION
The invention relates to a method for applying a diffusion barrier coating to filaments, specifically coated boron and silicon carbide filaments.
2. DESCRIPTION OF THE PRIOR ART
The demands for better performance of advanced aircraft and for reusability of space reentry vehicles have resulted in many new innovative materials. In recent years, the development of metal matrix composites (MMCs) has been vigorously pursued. The incorporation of high strength and high stiffness ceramic filaments into a ductile metal matrix can produce a composite material of low density that exhibits good mechanical properties at elevated temperatures. Various metallic alloys together with various forms of ceramic reinforcements have been investigated for chemical compatibility, composite mechanical properties, and failure mechanisms. MMCs have been shown to be satisfactory material systems if, in addition to high-temperature strength and oxidation resistance, the reinforcement and matrix are chemically compatible.
One matrix of interest is titanium. Titanium and its alloys are used extensively because of their good corrosion resistance and their high specific strengths at both room and moderately elevated temperatures. However, they are used only below 540.degree. C. (1000.degree. F.) since their strength drops rapidly above that temperature. By reinforcing titanium alloys with boron-based or silicon carbide-based filaments, large increases in specific stiffness and strength, both at room and elevated temperatures, can be achieved. However, due to the inherent high chemical reactivity of titanium metal, the properties of the Ti MMCs are limited by the extent of fiber/matrix interface reaction. The mechanical properties fall far short of the expected values.
The major reason for composites not achieving the expected mechanical properties is the detrimental chemical reactions that occur between the reinforcing fiber and matrix during fabrication of the composite and during service. The most serious problem is the formation of intermetallic compounds that seriously degrade the mechanical properties of the fibers.
There are three principal ways of reducing the deleterious interface reactions: (1) lower the composite fabrication or use temperature; (2) modify the chemistry of the matrix or reinforcement material; or (3) apply a diffusion barrier coating to the reinforcement. This invention addresses the development of a diffusion barrier coating process.
It is therefore an object of this invention to develop a successful diffusion barrier coating which reduces the reaction rate and also provides good bonding between the fibers and matrix. Another object of this invention is to develop a coating technology which can coat fibers continuously and uniformly as needed. It is also an object of this invention to Provide a diffusion barrier material which provides: a high melting point, chemical stability at high temperature, low density, compatible coefficient of thermal expansion (CTE) with that of the fiber and a low chemical reactivity at high temperatures with both the reinforcing fibers and the metal matrix. Magnesium oxide is one of the few materials that meets the requirements of being chemically stable and also has a good CTE match with the fibers of interest.